The present invention relates to the field of hydraulics. More particularly, this invention relates to a loop flushing circuit for a hydrostatic transmission. This invention provides pressurized lubrication for the rotating elements of the transmission. This invention was developed to provide lubrication flow to hydraulic components in an open case hydrostatic transmission module operating in a dry case environment. The invention does so without the need for external lubrication supply, hoses or external connections.
It is known that hydrostatic transmission modules can be incorporated into combination hydro-mechanical transmissions. In order to achieve a more compact design, the hydrostatic transmission module is often provided with an open casing and the hydrostatic transmission shares a common outer housing and reservoir with the mechanical portion of the hydro-mechanical transmission. In this situation, the hydrostatic transmission module must operate in a "dry case" or "non-full case" mode in which the hydraulic fluid that normally fills the hydrostatic transmission casing and lubricates the rotating elements of the hydrostatic transmission is drained to the common reservoir before it can lubricate the rotating elements. Therefore, the dry case mode creates a need for pressure lubrication of the hydrostatic rotating elements. In the context of this invention, the hydrostatic rotating elements can constitute various components including, but not limited to, pistons, slippers, cylinder blocks, shafts, gears and bearings. A reliable source and quantity of lubrication oil must be provided at all times, even when the hydrostatic transmission is in neutral. The rotating elements may still turn when the hydrostatic transmission is in neutral.
It is very difficult to design lubrication supply lines into the hydrostatic transmission module due to limited space and relevant movement of the module within the hydro-mechanical transmission. The hydrostatic transmission module is intended to be assembled as a separate module, then installed and mounted inside the hydro-mechanical transmission housing. Space surrounding the installed hydrostatic transmission module is minimal, making it difficult if not impossible to connect lubrication supply lines or hoses between the hydro-mechanical transmission housing and the hydrostatic transmission module. The hydrostatic transmission module may also move relative to the hydro-mechanical transmission housing due to the module being supported by elastomeric mounts within the housing.
Some closed loop hydrostatic transmission systems include an integrated loop flushing valve which automatically senses and drains a regulated quantity of oil from the low pressure side of the hydrostatic loop for cooling and filtration purposes. However, typical loop flushing valves block the flushing flow when the hydrostatic transmission is in neutral or when there is no pressure differential between the system pressure lines. See FIG. 1. Previously, the loop flushing oil from conventional flushing valves has been dumped into the hydrostatic transmission casing in full case systems or collected and sent to other parts of the vehicle hydraulic system in open case systems. In typical open case systems, two additional hydraulic connections between the hydrostatic module and the transmission housing are required: one for collecting the loop flushing oil and the other for the externally supplied lubrication oil.
As an alternative solution, lubrication has sometimes been provided internally from the charge circuit. This is a less desirable alternative because it results in higher power usage. Charge flow is typically at a higher pressure than necessary for lubrication, so using charge oil for lubrication wastes energy.
It is desirable to minimize the number of oil connections between the hydrostatic transmission module and the hydro-mechanical transmission to:
1. reduce cost; PA1 2. minimize assembly time and complexity; PA1 3. achieve a more compact design; and PA1 4. minimize potential paths for noise transmission, both mechanical and hydraulic.
Furthermore, the hydrostatic transmission module lubrication flow could be an additional flow requirement on the vehicle. A way to avoid or minimize the need for a larger hydraulic pump and its power requirement is desired.
Therefore, a primary objective of this invention is the provision of a hydrostatic transmission module having a loop flushing circuit to provide pressure lubrication to rotating elements or for any other purpose under all operating conditions.
Another objective of the present invention is the provision of a hydrostatic loop flushing circuit which provides flushing flow and lubrication when the hydrostatic transmission module is in neutral, that is, has no differential loop pressure or flow.
Another objective of the present invention is the provision of a loop flushing circuit which supplies pressure lubrication to hydrostatic components, such as a pump or motor, operating in a "dry case" environment without the use of any external hydraulic hoses or lines.
Another objective of the present invention is the provision of a loop flushing circuit which allows selection of the loop flushing flow rate based on lubrication needs in addition to the traditional criteria of loop cooling and contamination flushing requirements.
Another objective of the present invention is the provision of a loop flushing circuit which has minimal impact on power usage.
Another objective of the present invention is the provision of a loop flushing circuit which aids in achieving a more compact design of the hydrostatic transmission module and the hydro-mechanical transmission.
Another objective of the present invention is the provision of a loop flushing circuit which reduces costs, assembly time and complexity.
Another objective of the present invention is the provision of a loop flushing circuit which avoids the need for additional components and the connections that are potential paths for noise transmission, both mechanical and hydraulic.
These and other objectives will be apparent from the drawings, as well as the description and claims which follow.